The subject invention relates to ethylene based polymers which are foamed and cured to form low density closed cell microcellular articles.
It is well known in the art that heating a composition containing a copolymer of ethylene with vinyl acetate, a chemical crosslinking agent and a chemical blowing agent in a pressurized mold will crosslink the copolymer and decompose the foaming agent so that on opening the mold the composition expands into a closed cell microcellular foam. Articles such as flat shoe soles, foamed matting and recreation vehicle fenders are manufactured in this way.
During the molding process, the composition reaches a suitable degree of cure before the pressure is released and the mold opened. If the mold is opened before the necessary degree of cure is reached, the polymer network is not strong enough to contain the now gaseous foaming agent and the article collapses as the gas escapes from the molten mass.
Using the conventional ethylene/vinyl acetate composition employed in the manufacture of foamed articles in the past, mold time generally is about one minute for each millimeter of thickness of unfoamed material at about 165.degree. C. A mold time of 15 minutes is not uncommon. The cost of this molding process being a considerable percentage of the total cost of the molded piece, the ability to run a faster mold cycle would increase production rate and consequently decrease the cost of the foamed end product. It is not possible to use a higher temperature to accelerate the curing of the usual thick molded articles, any higher temperature tending to result in the outer portion of the composition becoming overcured and the inner portion undercured. To have maximum utility, the foam must be resilient but provide support, have good tear strength, abrasion resistance and be low cost.